1. Field of the Invention:
The present invention relates to a fine adjustment mechanism for a scanning tunneling microscope (hereinafter referred to as an STM), and, more particularly, to a three-dimensional fine adjustment mechanism which employs a cylindrical piezoelectric element.
2. Description of the Related Art:
Generally, STMs include a combination of a coarse adjustment mechanism and a fine adjustment mechanism, the coarse adjustment mechanism for holding a sample to be observed and the fine adjustment mechanism for retaining a probe. In such STMs, after the sample has been advanced by the coarse adjustment mechanism to the vicinity of the forward end of the probe, the probe is moved towards the sample by the fine adjustment mechanism while a voltage is being applied between the sample and the probe, until a tunneling current having a predetermined magnitude flows therebetween. Subsequently, the probe is moved along the surface of the sample by the fine adjustment mechanism, to detect the irregularities on the surface of the sample on an atomic scale utilizing variations in the tunnel current.
The fine adjustment mechanism for use in such STMs in general employs a cylindrical piezoelectric element, such fine adjustment mechanism being disclosed, for example, on pages 1688 and 1689 of Rev. Sci. Instrum., Vol. 57 (1986) by G. Binnig and D. P. E. Smith and on pages 145 to 153 of Surface Science, Vol. 181 (1987) by K. Besocke.
In any of these fine adjustment mechanisms, four electrodes are provided on the outer peripheral surface of a cylindrical piezoelectric element are separated from each other around the periphery of the cylinder, and an electrode is provided on the entire inner peripheral surface thereof. A probe is mounted on one end of the cylinder. The probe is moved in directions (X and Y directions) perpendicular to the axis of the cylinder by the application of voltages having opposite polarities to an opposing pair of the electrodes formed on the outer peripheral surface of the cylinder. Application of voltages between the electrode formed on the inner peripheral surface of the cylinder and the electrodes provided on the outer peripheral surface moves the probe in the axial direction (Z direction) of the cylinder.
It is known that scanning sensitivity, i.e., the rate at which the probe is displaced relative to the voltage applied to the electrode, of such a fine adjustment mechanism is determined by the piezoelectric distortion constant of the piezoelectric element and the dimensions of the piezoelectric element and the electrode. Therefore, a conventional fine adjustment mechanism has an inherent scanning sensitivity which is based on the structure thereof, and which cannot be freely changed.
As a result, a fine adjustment mechanism having a small scanning sensitivity is selectively employed when the surface of a sample in a small scanning area is observed in detail. Conversely, in order to observe the surface of the sample in a broad area, a fine adjustment mechanism having a large scanning sensitivity is selectively used. Thus, the choice of a conventional fine adjustment mechanism involves a problem in that a fine adjustment mechanism which suits a particular object of observation must be employed.
Further, if the waveform of a voltage applied to the electrode is varied, the probe of a fine adjustment mechanism having a fixed scanning sensitivity can be moved finely and coarsely. However, this technique requires a complicated, expensive voltage supply device for applying the voltage to the electrode.